1. Field of the Invention
The present invention relates to a printer in which a print head is moved in a reciprocating manner along a printing region defined by a paper guide surface and prints on recording paper transported along the paper guide surface through the printing region. The invention also relates to a method of automatically setting recording paper in such printer.
2. Description of the Related Art
Cash registers used, for example, as point of sale (POS) terminals are generally equipped with a printer for printing on roll paper. After printing, for example, the purchase price of merchandise, the amount received, and the amount of change returned, the printed roll paper can be cut or torn off and issued as a receipt.
A paper guide surface defining the printing region or printing position of the print head is formed at a midpoint in the transportation path of the roll paper in such a printer, and the print head is held opposite to the paper guide surface. More precisely, part of the transportation path is open to expose the roll paper to the print head and to enable printing by the print head on roll paper.
When the leading edge of newly loaded roll paper is transported along this transportation path during roll paper replacement, only one side is guided by the paper guide surface at this open part of the transportation path. The leading edge of the roll paper thus tends to curve to the open side, that is, toward the print head, to leave the transportation path and, thereby, to cause a paper jam. Paper jams occur particularly easily at this type of printing position when the leading edge of the roll paper is curled or folded.
To avoid such paper jams during roll paper replacement, a roll paper setting mechanism for a printer disclosed in JP 2-219672/1990 A is designed to close the open part of the transportation path on the print head side by means of a movable guide when the roll paper is replaced. When an auto-load switch is operated for roll paper replacement, the print head is tilted backward to increase the gap between the print head and the platen, and the movable guide is moved into the thus widened transportation path. This limits the transportation path to a path in which both sides are closed by means of said movable guide and the platen, respectively, and the leading edge of the roll paper is guided along this transportation path to the exit side.
Because a transportation path of which both sides are closed is temporarily formed by thus moving a movable guide, the leading edge of the roll paper can be reliably passed to the transportation path on the exit side. In addition, after roll paper replacement is completed, the movable guide is retracted from the position opposing the platen, the platen returns to a condition opposing the print head, and printing by means of the print head to the roll paper as it is transported along the platen is again made possible.
In this prior art, a mechanism for tilting the print head in a direction away from the platen, i.e., about an axis parallel to the direction in which the print head moves for printing, is required and space must be provided to allow for such tilting of the print head. This arrangement invites corresponding increases in the size and cost of the device.
In addition, print heads are normally designed to achieve an optimum printing operation when held at a predefined platen gap. Therefore, a tilting movement of the print head as explained above which changes the positional relationship between the print head and the platen that determines the platen gap is not desirable. If the return position of the print head due to backlash for instance is not accurately controlled a change in the platen gap will result. It is particularly necessary to maintain a constant platen gap when an ink jet head is used as the print head, in which case even momentarily moving the print head in a direction to retract it from the platen is not desirable.
Furthermore, there are also cases with a cash register where it is necessary to print on cut-sheet forms and other slip forms that are wider than the roll paper. Unlike the cut-sheet forms typically used in a corporate office, slip forms used as the recording paper in this case in supermarkets, convenience stores, and similar locations often have wrinkles, curls, or creases. As a result, even with slip forms the leading edge of the transported recording paper tends to separate from the surface of the platen or other part of the transportation path, resulting in a paper jam at that point similar to the situation that occurs when roll paper is replaced.
In order to facilitate setting slip forms easily and correctly in slip printers, such printers are normally equipped with a mechanism for switching paper feed rollers between a closed state engaging each other and an open state separated from each other, a paper stop and a mechanism for moving the paper stop. While applying the movable guide disclosed in JP 2-219672/1990 to such slip printer might be helpful to set the slip forms more correctly, providing respective mechanisms for moving or switching the guide, the paper stop and the feed rollers would increases the printer size. Further, controlling these mechanisms for setting a slip form would become rather complicated.